To treat hemorrhaging (e.g., escape of blood from a ruptured vessel), it is often necessary to quickly infuse a substantial volume of fluid, e.g. whole blood, plasma or blood substitute, so as to maintain an adequate blood volume and circulation. However, to preserve blood or blood products, such materials are typically refrigerated, and it is necessary to warm them before infusion so as to avoid shock to a patient's system. While it is desired to heat the infusate quickly, the temperature of the infusate should not exceed 40° C. to 42° C., since at higher temperatures, protein denaturation and red cell damage can occur. It is also important that any electrical power or potentials utilized in the heating process be effectively isolated from a patient.
Furthermore, hyperthermia (e.g., an individual's body temperature elevated above his or her normal range) as a treatment of tumors has been carefully studied and applied. Research has shown that high temperatures can damage cancer cells, usually without significant damage of normal tissues. Temperature and time may be interrelated with respect to tumor necrosis and risk of toxicity to normal cells. With longer time at hyperthermia temperature, more tumor cells, as well as healthy cells may be damaged. Tumor cells may not efficiently dissipate heat due to their disorganized and compact vascular structure; therefore, hyperthermia may cause the tumor cells to undergo apoptosis.
Hyperthermia may be particularly useful for reducing the risk of metastasis by targeting circulating tumor cells within the bloodstream. Cancerous cells may circulate within blood vessels (e.g., after surgical removal, cytoreduction surgery, de-bulking of tumor), attach to 9292473v1 endothelial cells lining the blood vessels, and form secondary tumors or metastatic lesions. However, a targeted treatment may not be possible for those circulating tumor cells. Hyperthermia via body cavities can be effective in treating with circulating tumor cells. While healthy cells (e.g., non-tumor cells) may survive the treatment due to heat transfer to the blood vessels, the floating tumor cells may not.
Fluid warmers often have a sensor connected to inflow tubing to detect if an infusate reservoir is empty. Typically, the sensor comprises a transmitter and a receiver (FIGS. 14A-14C), and measures velocity of ultrasound waves from the transmitter to distinguish between fluid and air, since ultrasound waves travel faster through fluids than air. The inflow tubing is located between the transmitter and the receiver. When the fluid level in the infusate reservoir is low (or zero), the pressure in the infusate reservoir and the inflow tubing can be lower than the atmospheric pressure. Therefore, the non-rigid inflow tubing deforms to minimize the pressure difference. The fluid out sensor measures this deformation.
The direction of deformation affects detectability by the sensor. In some instances, e.g., where the major axis of the elliptical cross-section of the collapsed inflow tubing is parallel to a line between the transmitter and the receiver, the sensor cannot detect the deformation due to lack of air in the ultrasound beam path.